Application Ser. No. 08/514,609, the '609 Application, teaches an electrically small contrawound toroidal helical antenna (CTHA) comprising a single conductor with two length portions in overlapping contrawound relationship to one another. Electrical currents in the individual length portions travel in opposite circumferential directions around the toroid, so that the net circumferential electric current around the toroid is effectively zero. However, because of the contrawound helical relationship, the associated circumferential magnetic current components created by the respective electric current components in each of the toroidal helical length portions reinforce, so that the resulting radiation pattern is similar to that of an electric dipole coincident with and centered along the major axis of the torus. In other words, the resulting radiation pattern is strongly linearly polarized in a direction parallel to the major axis of the toroid. Depending upon the construction of the antenna, particularly the aspect ratio of the underlying torus form and the number of helical turns, other polarization components may also be present.
The '609 Application, incorporated by reference herein, teaches a schematic symbolism for representing generalized helical and generalized toroidal helical windings as solid or dashed lines, the former representing a left had pitch sense, the later representing a right hand pitch sense, wherein the axial direction of the associated magnetic current and the projected axial direction of the associated electric current are the same for a right hand pitch sense helix, and opposite for a left-hand pitch sense helix. The radiation pattern of an electromagnetic antenna can be related to the effective electric and magnetic current distributions created by the antenna. For example, a uniform ring of magnetic current with no associated electric currents corresponds to the radiated electromagnetic field distribution of an electric dipole antenna. Furthermore, a uniform ring of electric current with no associated magnetic currents approximates the radiation pattern of a "Smith Cloverleaf" antenna. The radiation pattern for a particular set of current distributions can be determined by either simulation or measurement.
In an exemplary mode of operation, the antenna is operated at a frequency such that the circumferential length of the antenna is one half of an electrical wavelength. The slow wave properties of the contrawound helix make the corresponding physical length shorter than the free space wavelength according to the associated velocity factor, which depends upon the associated underlying helix geometry.
One limitation of the above described contrawound toroidal helical antenna is that the bandwidth of the antenna is about 10%. Accordingly, for broadband applications for which a greater bandwidth is required, a plurality of contrawound toroidal helical antennas are necessary wherein the respective resonant frequencies of the antennas are separated from one another in such a manner that for a given frequency of operation within the associated frequency band, the one of the plurality of antennas having the lowest VSWR at the transmission line side of the associated impedance matching network is used for transmitting or receiving the given signal. Accordingly, as illustrated in FIG. 76 of the '609 Application, a broadband signal may be directed to or extracted from the appropriate antenna using a multiplexer. In another embodiment, individual transceivers could be adapted to each antenna element. In yet another embodiment, a multiplexer may be used to interface one transmitter with a plurality of antenna elements, and individual receivers may be operatively coupled to each of the antenna elements, the outputs from which are combined so as to form a composite received signal.
As illustrated in the above referenced FIG. 76, the individual antenna elements are concentrically co-located about a common central axis. This has the advantage of providing for phase symmetry of the resulting transmitted waves with respect to the common axis. However, one problem with this arrangement is that transmission line sides of the respective impedance matching networks cannot be interconnected to a common signal port without incorporating transmission line segments between one or more of the impedance matching networks and the common signal port because of the physical separation between the antenna elements. These transmission line segments introduce phase delays in the signal that are a function of frequency, which precludes the direct interconnection of the transmission line sides of the respective impedance matching networks so as to achieve natural broadband operation a the common signal port.
Another limitation of the above described contrawound toroidal helical antenna is that the antenna input impedance is generally significantly different from the characteristic impedance of typical transmission lines, which therefore requires the use of an associated impedance matching network in the signal connector. More particularly, for a relatively wide bandwidth resonance condition, the input impedance of the antenna is generally from 1 to 3 K.OMEGA.. By contrast, typical transmission lines have an impedance of 50-300.OMEGA..